1. Field of the Invention
The invention relates generally to diagnostic instrumentation for semiconductor wafer processing equipment and, more specifically, to a capacitive probe for in situ measurement of the DC bias voltage accumulated on a semiconductor wafer while being processed in a semiconductor wafer processing system.
2. Description of the Background Art
When monitoring semiconductor wafer processing systems, it is important to accurately measure the DC bias voltage on the wafer. One method of measuring this value is with DC bias "plugs". For example, in the P5000 metal etch chamber, manufactured by Applied Materials of Santa Clara, Calif., DC pickups or "plugs" composed of a graphite or silicon carbide sleeve surrounding an aluminum shaft are placed in direct contact with the plasma. Assuming a near zero potential drop across the plasma, it is possible to estimate the DC bias on the wafer. Unfortunately, these types of "plugs" degrade and represent a consumable material that increases the maintenance costs of the chamber. Additionally, as the "plugs" are consumed, contaminants are released into the processing environment. As such, replacing the "plugs" with a device that measures DC bias voltage without contacting the plasma is preferred.
An improved measurement technique is to measure the amplitude of the RF voltage (e.g., peak-to-peak voltage) at the pedestal and assume the wafer is at the same potential. The measurement of this value is rectified to estimate the DC bias on the wafer. However, this also proved to be a difficult DC bias measurement because of the high level of RF noise proximate the pedestal assembly that penetrates into the electrical measuring circuitry. Additionally, this technique requires substantial modification of the chamber to facilitate coupling a probe and its associated circuitry to the pedestal.
Therefore, there exists a need for a device that can estimate DC bias voltage at a wafer during processing without experiencing the limiting and erroneous effects of RF noise and/or physical degradation of voltage probes that contact the plasma.